Elastic spectroscopy of porous and anisotropic materials

Lightweight materials are of enormous importance in particular in aerospace applications. Usually these materials are composites, i.e. frequently they consist of a matrix, which is reinforced by fibers or particles. Due to this processing, the materials are usually anisotropic and additionally have a considerable porosity. This is not restricted to technical materials, biological materials such as bone or wood are also anisotropic porous materials, where the porosity (such as blood vessels or tracheids) is necessary for the nutrition of the tissue. To calculate the general mechanical properties of the composite by Finite-Element-calculations in a technical application, a complete set of elastic constants is required. The measurement of these constants faces considerable problems. If there exists no previous knowledge on the material, combining mainly analytical methods such as Timoshenkos resonant beam method with numerical procedures (resonant ultrasound spectroscopy) is favourable. By using sophisticated theoretical approaches, additionally the porosity should be taken into account. The main goal of the project is to obtain elastic data from specific materials and to calculate therefrom the elastic properties of the dense material, the porosity and orientation by using the different theoretical models. To test the validity and the limitations of the theoretical approaches, these data should then be compared to the microstructure of specific materials characterised by appropriate methods (pore size distribution by digital image processing of light microscopy, nitrogen absorption and small-angle X-ray scattering). By this, the elastic properties of the bulk material ("the material`s quality") and the oriented porosity ("the material`s architecture") should be obtained simultaneously.

Highly anisotropic and porous materials are of particular interest for a number of applications, e.g. a high specific strength in certain directions, light-weight structures for automotive and aerospace industries, which are nevertheless stiff, materials with excellent damping behaviour for noise reduction to protect the environment and catalysts or chemical reactors due to the high surface to volume ratio. A prerequisite for the technical use is the knowledge of the elastic properties. The elastic moduli and/or elastic constants are basic for the design of real workpieces in construction to calculate the actual deformation under load by using Finite-Elements. Within the frame of the project, the "resonant beam technique" was further developed to measure these elastic moduli and/or the elastic constants of materials with strong anisotropy and extremely high porosity. The first progress was the theoretical solution of the shear correction factor for anisotropic beams, which is based on the actual stress distribution across the cross-section. This theoretical solution laid the basis for the further development of the experimental equipment. It could be shown that with this method the elastic moduli of ceramic and metal foams as well as ceramics covering an extremely wide range of porosity could be determined. Another interesting feature is the possibility of measurements in dependence on the temperature up to 2000 C. This allowed the measurement of the anisotropic elastic properties of the nozzle of the European space rocket Ariane V. The resonant beam technique is now subject to become an European standard - a draft for this standard was sent to the responsible standardization committee of the European Union (CEN TC184 SC1).